1. Field of the Invention
The present invention relates to an image forming apparatus which is capable of producing not only a normal image but a reversal image.
2. Description of the Prior Art
In some of conventional image forming apparatuses having an electrophotographic reproducing process, there are provided a normal image producing mode for producing a positive image from a positive original or a positive film and a reversal image producing mode for producing a positive image from a negative original or a negative film. The image forming apparatuses of this kind employ therein either of the following image forming methods to carry out these two image producing modes.
One of such image forming methods requires an electrostatic latent image support member which is sensitive to both positive and negative polarity and two electrifying chargers disposed in the vicinity of the electrostatic latent image support member for charging the surface of the electrostatic latent image support member in the positive and negative polarity, respectively. In this method, the electrostatic latent image support member is initially charged in either positive or negative polarity in accordance with the selected image producing mode. Identical toner is then supplied to either an electric charge holding portion or an electric charge erased portion of the electrostatic latent image formed on the support member.
Another image forming method requires an electrostatic latent image support member which is sensitive only to either the positive polarity or the negative polarity, a developing device having therein toner of the positive polarity, another developing device having therein toner of the negative polarity and two transfer chargers having respective discharging characteristics in different polarity. In this method, toner of the positive or negative polarity is supplied onto the surface of the electrostatic latent image support member in accordance with the selected image producing mode. Furthermore, either one of the transfer chargers is selectively used in accordance with the polarity of the supplied toner.
However, a multi-color image forming apparatus having a plurality of developing devices is provided in its limited space along the periphery of the electrostatic latent image support member with various devices required for executing the electrophotographic reproducing process. Accordingly, the apparatus does not have enough space for the two transfer chargers or electrifying chargers. This is particularly conspicuous in a small-sized copying apparatus, printer and multi-color copying apparatus.
Furthermore, in the image forming method of the former, it is necessary to switch the polarity of developing bias voltage which is applied to a developing sleeve accommodated in a developing device, rendering a power device to become undesirably large.
In the method of the latter, because the two developing devices accommodate respective toners which are different from each other in polarity, an image produced is occasionally subjected to fog due to the toner supplied from the developing device not in use.